Caribbean coral reefs result of mass extinction, rise of isthmus
Research suggests extinction may be gradual and unpredictable
mongabay.com
March 12, 2007

Extinctions that resulted from the formation of the Panamanian isthmus were delayed two million years according to a new study by researchers at the Smithsonian Tropical Research Institute, Scripps Institution of Oceanography and London's Natural History Museum. The findings may have implications for global species extinction and evolution.

Three to four-and-a-half million years ago, the volcanic rise of the Panamanian land bridge between North and South America divided a once great ocean into the Caribbean Sea and Pacific Ocean we know today. The closure of the isthmus eventually led to the development of coral reef ecosystems in the Caribbean and the formation of a nutrient-rich, upwelling current in the Pacific. It also caused the so-called "Great American Interchange" where terrestrial animals migrated between the once separated continents, as well as a mass extinction among some marine species. The new research, published in the March 12 issue of the Proceedings of the National Academy of Sciences, focused on this extinction of marine species.

The Caribbean side of Panama is characterized by crystalline waters and coral reefs whereas the Pacific coast of Panama generally lacks coral reefs and is murkier due to upwelling which brings cooler, nutrient-rich waters from the depths of the Pacific.

Aaron O'Dea at the Smithsonian Tropical Research Institute in Panama City, Panama.

Felix Rodriguez, a researcher at the Smithsonian Tropical Research Institute in Panama City, Panama, examining cupuladriid fossils. Aaron O'Dea says that cupuladriids can be used as a proxy for strength of upwelling. "Before closure fossil cupuladriids show seasonal changes in growth - much like tree rings - as the colony grows zooid size changes and leaves a record of seasonality as the colony expands. After closure, this pattern does not occur in fossil cupuladriids and so with this approach we can pinpoint the end of seasonal upwelling in the Caribbean."

Analyzing fossil evidence and comparing it with contemporary marine life in waters around Panama, the researchers found significant shifts in species distribution after the formation of the isthmus, including the rise of corals and algae in the nutrient poor waters of the Caribbean and filter-feeding clams and mussels in the Pacific. Aaron O'Dea, postdoctoral fellow at the Smithsonian Tropical Research Institute (STRI) and Scripps Institution of Oceanography, said that the changes reflect the availability of nutrients in the water.

"Changes in the body size of fossil cupuladriid bryozoans indicate the strength of upwelling; the types of fossil foraminifera in the samples tell us the depth of the water, the percent shell material and mud in the sediments give us an idea of the amount of food in the water column."

The blockage of the nutrient-replenishing current from the Pacific caused stagnation in the Caribbean, eventually triggering the rise of complex communities including coral reefs and algae where competition over scarce resources was fierce. Meanwhile, the upwelling in the Pacific brought a wealth of nutrients from the depths of the ocean, producing a greater abundance of life but less species richness. Species that couldn't adapt to the environmental change, like Carcharocles (Carcharodon) megalodon -- a giant shark that once roamed the expansive ocean -- gradually went extinct.

"The existence of the C. megalodon shark shows how the Caribbean was very different to how it is today - that there was enough primary productivity in the Caribbean to support such large animals," O'Dea told mongabay.com in a meeting at a STRI lab in Panama.

Yadixa Delvalle, an indigenous Kuna who works in traditional dress, at STRI. Yadixa worked a lot on this particular project, according to O'Dea.

Aaron O'Dea collecting samples in Panama.

Looking for fossil evidence on Finger island.

Delayed Extinction

Work by Ken Johnson and Jon Todd of the Natural History Museum in London, also co-authors of the paper, found that up to 50% of coral and snail species went extinct between one and two million years ago, a full two to three million years after the closure of the isthmus. The researchers aren't sure what was responsible for the tome-lag between environmental change and extinction.

"We don't start seeing extinction rates really increasing for another 1 million years. What is most remarkable is that most of the organisms that went extinct were those that liked high productivity environments, which had already disappeared long ago. Why did it take so long for them to perish? What tipped the scales?" asked Jeremy Jackson, professor at Scripps and STRI Staff Scientist emeritus. "Most paleontology studies report the length of time that an organism is present in the fossil record but that doesn't tell the whole story. Looking at the abundance of organisms helps because you can see their decline long before they go extinct. Extinction is forever, and it remains unclear how long new ecological conditions must persist before extinction occurs."

"This time lag suggests that something other than environmental change per se was involved in extinction that does not occur as a single event," the authors wrote, though O'Dea added that the delay, while still mysterious, may have implications for present-day biodiversity loss.

"The answer is probably that species underwent severe reductions in their abundances and geographical ranges but managed to cling on until some sort of threshold was reached. We may choose to take this as a warning because right now we are seeing similar reductions in ranges and abundances of many animals and plants due to anthropogenic influences, but we still don't know where the thresholds are that lead to irreversible extinction."

Other scientists have warned that we are currently in the midst of another extinction event, one that is of our making due to deforestation, the introduction of alien species, and overexploitation of certain species. There is concern that rapid climate change could spark further species loss.

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